1) Log in to your WHM.
2) Look for a link that says "Create a New Account" under the Account Functions
header.

3) You should see something like this:

From here enter the domain the client requested as well as the username and
password they requested in the proper field. From here also choose the plan they
requested (IE starter, standard, etc..) If they did not request a domain on
their order you can set it up as TheirUsername.YourDomain.com. Make sure to
replace TheirUsername with the actual username they request and YourDomain.com
with your (the reseller) actual domain.
4) After this click create and the account will be set up. From here you can send
a e-mail to them with the account info or anything else you have planned.